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Building on their team’s previous work with soy-containing diets and allodynia and hyperalgesia in rats, Shir et al.
designed a study to investigate whether timing of a soy diet is critical for suppressing sensory disorders produced by nerve injury. In groups of 8–10 each, 12 groups of male rats were fed according to seven different regimens. Diets consisted of either a soy-based or a nonsoy-based food product. In one group, soy was fed to rats 14 days before nerve injury and 14 days thereafter. In other groups, rats received soy food before nerve injury but not afterward; no soy before or after induced injury; or soy foods only after the injury. Behavioral tests, assessing sensitivity to tactile stimuli, mechanical pain, or heat, were conducted 1 day before nerve injury (unilateral partial sciatic nerve ligation), and after injury on days 3, 8, and 14. After partial sciatic nerve ligation injury, rats in all groups had mechanical hyperalgesia, expressed as a significantly increased response duration to pin prick compared with baseline. Rats fed soy diets before partial sciatic nerve ligation injury exhibited significantly blunted postoperative levels of allodynia and hyperalgesia. However, there was no additional suppression of neuropathic pain seen in groups fed the soy diet both preoperatively and postoperatively, and the rats fed nonsoy diets before surgery combined with soy diets after surgery also did not exhibit any pain suppression. The protective effects of a presurgery soy diet seem to be short-lived because switching to a nonsoy diet 15 h before injury deleted the pain suppressing effect. The specific factors in the diet responsible for these changes remain to be identified.